1. Field of the Invention
Broadly speaking, this invention relates to microwave communication. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for preventing accidental signal transfers between cross-polarized channels in a microwave communication system.
2. Discussion of the Prior Art
To reduce the bandwidth requirements of microwave communication systems it is known to employ cross-polarized radio channels operating on the same carrier frequency. By careful system design, the discrimination between such cross-polarized channels can be made greater than 20 dB, which is more than adequate for digital transmission systems.
For example, the 18 GHz digital radio system known in the industry as DR-18, derives seven working channels and one protection channel in each direction using only four carrier frequencies. Each such orthogonally polarized channel can carry up to 4032 voice circuits using pulse code modulation (PCM) to a rate of 274 Mb/s.
A problem arises, however, when one of the working transmitters fails. Normally, when the path loss at a particular frequency becomes excessive, for example due to heavy rain or fog, the violation monitor associated with the far-end protection switching equipment will detect an excessive number of parity violations and/or an "out-of-frame" condition and will initiate a switch to the protection channel. However, if only one of the two transmitters operating on a particular frequency fails, the corresponding far-end receiver will detect the cross-polarized signal transmitted by the other transmitter. Although this undesired signal will be some 20 dB lower than normal, it is, nevertheless, well within the 40 dB fade margin designed into the DR-18 system. Thus, the substantially lower amplitude of the received signal will not, per se, initiate a protection switch at the far-end receiver.
An additional problem is that because the bitstream received from the distant transmitter has the correct format, it will propagate through the system on the failed channel and be accepted by the far-end violation monitor, which, thus, will also fail to initiate a protection switch to the spare channel. Furthermore, since the digital signal is intelligible after demultiplexing, a loss of transmission security will result on the orthogonally polarized channel.
One solution to this problem would be to provide a lockout circuit for each receiver in the system. This lockout circuit would compare the AGC voltage from its own receiver with the AGC voltage from the receiver of the corresponding orthogonally polarized signal. When the difference in AGC voltages was sufficiently large, the regenerator of the failed channel would be locked, effectively preventing detection of the transferred digital bitstream.
With this arrangement, each receiver in the system requires its own lockout circuit as well as an interconnection to the corresponding orthogonally polarized receiver. Since a typical microwave route will include several intermediate receiver-transmitter pairs, the possibility of good channel being erroneously blocked by a failure of the lockout circuit is quite high.